Renewable Energy is of increasing importance to national and global energy policies and methods for harnessing renewable energies are attracting increasing amounts of investment. Marine renewable energy devices are an important part of the energy mix, particularly for countries with a good marine resource. Such marine renewable devices include tidal energy devices, wave energy devices and off-shore wind turbines.
Wind turbines (or wind-powered electricity generators or other wind energy capture devices) are typically multi-blade devices (usually three blades) mounted on a tower which is fixed in place with a monopile (or, increasingly for larger sized turbines, tri-pile foundations). The wind turbine tower is subject to various stresses arising from the movement of the substantial blades and from exposure to variable wind conditions. These stresses can cause movement of the towers which can then cause the towers to vibrate. Such vibrations can cause structural failure of the wind turbine or lead to increased maintenance requirements. Monopile mounted devices vibrate according to the modes of natural frequency of the system (especially the first and second modes), which is a function of the height of the monopile and the weight distribution across the device (as well as other factors). It is known to minimise the vibrations in a system and to dampen the vibration of a wind turbine tower by suspending a certain mass within or in association with the tower at a certain height (which damping mechanism may capture the energy of vibration by oscillation of the suspended mass). Off-shore wind turbines, however, which are monopile mounted are susceptible to sea-bed scour (e.g. by tidal variations or due to stormy weather), which removes an area of sea-bed about the base of the wind turbine monopile. This can, in effect, cause the height of the wind turbine to vary according to the degree and severity of sea-bed scour with the result that the optimal position and configuration of vibrational damping systems may vary (with resultant increase maintenance to review and vary damping configurations and/or increased risk of structural damage to the device).
It is therefore of utmost importance to ensure that the scouring about the monopile base and related structural integrity, vibrational variations, maintenance and possible structural damage are minimised.
Tidal energy devices designed to capture the regular and predictable tidal energy may be sea-bed mounted, optionally via an anchor or one or more sea-bed piles. Since tidal energy devices are typically located in areas of high tidal currents (to maximise energy capture), their anchor arrangements are particularly susceptible to tidal scour which may cause loosening of the anchor or sea-bed mount and resultant movement and damage or hazard from the device. The process of scouring may also result in inefficiencies in the operation of tidal energy devices.
Again, ensuring that the scour about seabed anchors for tidal or wave energy capture devices is important to prevent instability in the system and resultant damage.
Methods and materials for seabed scour protection exist and attempts have been made to address scoured seabed problems associated with seabed mounted structures.
Around wind turbine monopiles without scour protection, scour depths about the monopile of more than twice the diameter of the monopile have been observed, which presence of scouring requires monopiles of an extra 8-10 m in length to be utilised to ensure adequate structural stability and risks the vibrational variations, maintenance costs and damage referred to above. Scour can also leave a cable leading from a wind turbine exposed to turbulence and damage.
Scour protection for seabed-mounted off-shore wind turbines is a recognised problem and a significant cost of construction of such wind turbines is in the scour protection systems, which are recognised as being inadequate.
Typical offshore wind turbine scour protection may be approached in two ways: installing the wind turbine and then repairing scour that forms around the base (dynamic scour protection); or forming the scour protection before or immediately after installation (static scour protection). The scour protection typically takes the form of armour protection (e.g. a layer of rocks or large concrete elements), typically with rocks of diameter of the order of 50-100 cm, and a filter layer beneath of smaller stones or rocks having a diameter of the order of 10 cm. The protection may be of a meter or two in depth and may extend several meters in radius from the monopile.
However, it has been found that these traditional methods result in local scour at the extreme boundary of the scour protection area and the surrounding seabed, which can lead to undermining of the limits of the scour. It has also been found that the foundation of the scour protection has lowered about the base of the monopile, due to erosion of the sea-bed beneath the layer of scour protection (see, for example, Hansen et al, “Scour Protection around Offshore Wind Turbine Foundations, full-scale Measurements”, EWEC 2007). Erosion about the monopile or tripod/tri-pile foundations of wind turbines has been shown to have a significant impact on the natural frequency of vibration of a wind turbine, particularly on the second mode of vibration (see, for example, M. B. Saaijer, “Tripod support structure—pre-design and natural frequency assessment for the 6MW DOWEC”, doc. No. 63, TUD, Delft, May 2002).
There have been a number of efforts to improve scour protection. Fronded concrete mattresses have been proposed and found use in deepwater oil installations. However, this solution suffers from a number of disadvantages including a high cost of installation, the development of local scouring about the boundary of the concrete mattress, depression of the elements of the concrete mattress due to erosion of the seabed beneath and the inadequate performance of such devices in high energy shallow waters. Fronded fibre or textile mattresses are utilised about oil platform supports. These devices have fronds that are buoyant and extend upwards from the textile mat. Whilst having some effect in relatively low sea current environments, they suffer from certain disadvantages. In particular, in high current environments, the fronds are forced to a very shallow angle to the mat and lose a significant amount of their sediment trapping capability. In addition, in strong current flows, the seabed material about the edges and beneath the mat can be undermined leading ultimately to disturbance of the mat anchors and the fronded mat being unsecured and moving away with the currents.
There have been attempts to bypass some of the consequences of scouring. WO-A-2008/151660 describes a method for containing a cable leading from a wind turbine to shore (typically) which prevents the cable from being damaged from exposure in the event of scour about the base of the monopile. The tubing arrangement provided can be fed from above the water and is not distorted by scouring about the monopile (as is typical in a conventional J-tube arrangement), by being hingedly connected to a rigid tube leading outwardly from the monopile base into the seabed. Whilst this provides a solution to a problem caused by seabed scour, it does nothing to address the underlying scour problem.
It would be desirable to provide a method and/or apparatus for inhibiting or repairing scour about a sea-bed mounted foundation such as a monopile, e.g. in association with an offshore wind turbine, or anchoring of other marine renewable energy device, which method and/or apparatus overcame the aforementioned problems in a cost effective and readily applicable manner.